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MG MGB Technical - B Series Cylinder Heads

I am new to the MG world and although I have owned BMC A series engined cars I would like to know what is theafe amount you can shave of a B series Magnette head in order to raise the compression, and how much shaved of will produce what ratio.

Thanks

Tony Oliver
A G Oliver

Tony, you've waited a long time for a reply on this threrad so I'll stick my neck out then you will get a barrage of replies.

As I remember it these engines 1489 and 1622cc were 8.3/1 ratio. I'd be very reluctant to take anything off the head as you will soon run into problems with pre ignition, running on etc. These engines needed a 98 octane fuel to perform properly and if you raise the ratio you will make it even more fuel critical. Presumably your idea is to increase the power output but there are many other ways of doing that which you will find on these archives. Be careful though or you will land up with something that sounds like an MGB and I don't think that would sound too good coming from a Magnette !!
iain.mackintosh

Thanks Iain

I was hoping to increaser power and have always been tols that increasing the comp ratio was the way to go. I will take yur advice on board when I finally overhaul the engine.

Tony

PS

I am an ex "POM" from Cheshire. how is it over there these days. I have been on Oz since '68.
A G Oliver

Have to disagree with Iain on this one. Early B with the beaked head used 8.8, and it was common to fit 9:1 pistons to MGA/Magnette with no problems, and we did not have to run top grade fuel (Sunoco 260 back then). The MGB owners book does say to use 98 Research for 8.8, and minimum 93R for 8.0, but, It was easy to get then,and it was your money (but not as much of it as today!) The factory comp manual says to use 100R octane at 9.5 for racing, and they further say that 100R octane is suitable up to 10.6. Vizard says that A series can run as high as 14.4 on 102 Research. 9.3 to 9.5 should be fine with today's premium (about 98-100RON in the US) if all else is right, and gives much better power and economy.
FRM
http://www.usachoice.net/gofanu
FR Millmore

Am fully aware that 100 octane fuel is not readily available that's why I said to be wary of raising the comp ratio much further especially re your comment on the factory recommendation of 100 octane at 9.5/1. You say these engines were 8.8/1 in which case there is little point in removing metal from the head face but I am fairly sure they were 8.3, perhaps Tony can verify if he still has the handbook. Had a Riley 1.5 at the time and it was certainly 8.3. I'd still be reluctant to remove metal as the head may already have been skimmed or may need skimmed at some future point.
Remember we are talking about a three bearing engine here and we don't want to knock the stuffing out of it. Improvements could be achieved by other methods such as gas flowing, replacing that air cleaners with K&Ns, change of camshaft and then exhaust system improvements. Seems then it's over to an 1800 unit.

Wow Tony, you've been gone a long time, I'm from the Scottish Highlands but everywhere seems to be much more crowded, more traffic and generally a faster pace of life. Much of the old industry has moved of course and that's a great pity but a sign of the times. We just can't compete against many countries who have more competitive labour rates. Still, not a bad part of the world to live in.

All the best, Iain
Iain MacKintosh

I did not say these (Magnette) engines were 8.8, but was giving an example of a similar engine with higher compression - the early (MG) B -sorry I didn;t write it all out. Magnette engine was 8.3.
I was further making the point that if 100RON was suitable for 10.6, then it certainly was not required for 9.5; and further, that the factory were being free with your money in these recommendations and the engines probably did not NEED 100RON. They further say to use minimum 93 for 8.0 engines, "with preference to 95/97"!
I don't know what Octane numbers are in current use in the UK, or other places like Australia, but in the US we now have the "average" of Motor and Research. (The book numbers quoted are Research) Research is something like 8-10 points higher than Motor, so US 94 is something like 98-99 Research, and 92 is 96-97. Exact numbers may vary by refiner, blend, season, etc.
All of the above should make it clear why I think that up to 9.5:1 should be OK on today's "Premium" fuel, and there are all manner of engines running in this range on this fuel today. As to knocking the stuffing out of it, everyone of the mods you suggest will do the same, but only raising compression increases the actual thermodynamic efficiency of the engine, and it is relatively cheap, especially if done at rebuild. These are pretty tough engines, and it is likely that the added power will only be used part-time, but the added economy will be always welcome. BTW, Vizard says that one point of compression in this range is worth about 2-3% power, and 4-6% economy, a bit more if a hotter cam is used.
FRM
http://www.usachoice.net/gofanu
FR Millmore

Tony You asked what you can safely take off a B series head. The MG performance hand book states .062" is safe as far as phisical strength and alters the CR on an MGB from 8.8 to 9.5:1,it would be a little different on your car but not much. I agree with the others though that you need to know what it is now before you start skimming the head (it might have some off already)If you end up with too high a comp you might need to retard the spark and lose more than the power you gained. Go no more than 9.5 after you do the sums. Denis
DENIS

Thanks to all for your input. I have previously gas flowed A series heads on Minis and intend to do the same to the Magnette, I just wondered what experience others had had with head shaving. I was always told that raising the comp ratio was a "freebie" when extra power was required.

Just to put you all in the picture, my engine is 8.3/1 according to my manual and I have presently got "pancake" filters fitted to the carbs.

An other "cheap" power tweaks would be very much appreciated. I used to have a copy of Vizards book for engine tuning, but foolishly believing I would never be silly enough to play with cars again, I threw it out many years ago.

Thanks

Tony
A G Oliver

Tony,

Vizard's book won't help with the B series head or engine.

About the only thing they have in common with the A series is that they have the same number of inlet & exhaust ports.

Get Peter Burgess's book and learn anew.
Chris at Octarine Services

If you're planning on rebuilding an engine, and not concerned with 100% originality, then consider a rebuild and install of an 1800 MGB motor. The later 5 main ones in particular are somewhat cheaper to rebuild, and offer an instant and easy power increase.
DLD Densmore

This thread was discussed between 15/09/2004 and 27/09/2004

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